One scheme for reliability-based design that is growing in popularity for civil and naval applications is the load and resistance factor design (LRFD). Our goal in this research is the development of a simulation to predict the remaining strength of structural composites subjected to variable fatigue loading and environmental exposure. The results of this simulation can then be used in LRFD to determine appropriate material factors of safety for engineering design applications. The work so far focuses on modeling the response of the material to fatigue damage only. A general phenomenological modeling approach is described and applied in two experimental studies using E-glass/vinyl ester composite materials. Strength distributions are modeled using Weibull statistics and residual strength is modeled using a strength-life equal rank assumption and a Monte-Carlo style simulation.

The model provides good residual strength distribution fits to constant amplitude fatigue data and worked well for ordered block spectrum loading using a 735,641 cycle, 22 stress level spectrum. However, applying a randomized spectrum produced unexpected results with every specimen failing after 200,000 to 400,000 cycles while the model predicts identical residual strength when compared with the block loading case. This work points to a need for focus on developing a better understanding of load order impacts in design of composite structures based on constant amplitude fatigue tests. A future approach toward more detailed micro-mechanics fatigue damage modeling is suggested to enable better modeling of residual strength of laminates subjected to random loading fatigue.